In a system driven by a synchronous motor, such as a machine tool, the drive unit of the synchronous motor is required to be immediately stopped for safety reasons in the event of a failure of the synchronous motor. In the event of a failure, such as emergency stop or blackout, the servo amplifier performs a power-generating brake by short-circuiting the input terminal of the synchronous motor with dynamic brake (hereinafter, “DB”) resistance. The DB is configured by a resistor, and converts electric power collected from the electric motor to thermal energy through the DB circuit. Thus, depending on the operating condition of the synchronous motor, the rotation energy of the synchronous motor might exceed allowable energy of the DB resistor of the servo amplifier, bringing a possibility of damaging the DB resistor by heat.
When immediately stopping a synchronous motor using a DB circuit, if the rotation energy of the synchronous motor is assumed to be completely consumed by the DB resistor, the following formula (1) must be satisfied to protect the DB resistor:Em<Ea  (1)where, Em [J] is the rotation energy of a synchronous motor, and Ea [J] is the allowable energy of the DB resistor.
Further, Em is given by the following formula:Em=0.5×J×ω2  (2)where, J [kgm2] is the inertia of the synchronous motor and the inertia of a driven object; ω [rad/s], the speed of the synchronous motor.
In a system driven using a synchronous motor, such as a machine tool, particularly with a driven object including a rotation table which mounts a variety of workpieces, the inertia of the driven object changes depending on the workpiece. As the rotation energy of the synchronous motor is determined by the inertia of the driven object as indicated by the formula (1), it is necessary to accurately estimate the inertia of the driven object.
Thus, there has been proposed a method of estimating inertia. For example, according to the method described in JP-A-2010-148178, the time constant of acceleration/deceleration and a speed gain can be optimized using the estimated result of inertia. However, in general, it is not easy to accurately estimate the inertia of a driven object, and, thus, whether the above formula (1) is satisfied or not is often difficult to judge.
Thus, there has been proposed a method of protecting a DB circuit without calculating inertia. In a conventional technique described in JP-A-2002-369564, the DB circuit is protected by calculating energy consumption of the DB circuit based on the speed of the synchronous motor at the time when operation of the DB circuit is started and time elapsed until the synchronous motor has decelerated to a predetermined speed.
Alternatively, another method of protecting a DB circuit without calculating inertia has been known. The conventional technique described in JP-A-2007-174729 protects a DB circuit by calculating energy consumption of the DB circuit based on the speed of the synchronous motor, the DB resistance, and the induced voltage constant of the synchronous motor.
In the methods described in JP-A-2002-369564 and JP-A-2007-174729, while inertia of a driven object is not required to be calculated, energy consumption of a DB circuit is consecutively calculated at sampling periods from the start of operation of the DB circuit. Then, only when the energy consumption of the DB circuit exceeds the allowable value, the operation of the DB circuit is terminated and the motor becomes a free-run state, which brings a problem where a distance until decelerating and stopping becomes long or time required for decelerating and stopping becomes long.
In an attempt to protect the DB circuit of a servo amplifier incorporating a DB resistor, the present invention aims to provide a controller for synchronous motors, which estimates the inertia of a driven object which is connected with the synchronous motor and limits the speed (maximum rotation number) of the synchronous motor using the estimated result.